A missile complex or missile silo was a structure in which missiles were housed ready for launch if needed. Such missiles included nuclear missiles or antimatter weapons.
On Earth, missile complexes were part of the military infrastructure associated with World War III. After the war, Dr. Zefram Cochrane re-utilized one such site in Montana as a launch facility for his his Phoenix ship, itself a modified nuclear missile. (Star Trek: First Contact)
U.S.A.F. markings within the complex would seem to suggest that this facility was originally owned by the United States Air Force.
On Otrin's homeworld, missile silos housing defensive antimatter weapons were built after the Friendship 1 probe introduced antimatter technology to the locals. (VOY: "Friendship One")